1. Field of the Invention
The present invention relates to a magnification observation apparatus such as a digital microscope and a microscope that images and displays a magnified image, and a method for photographing a magnified image.
2. Description of the Related Art
A magnification observation apparatus that magnifies and displays a subject such as a sample including a minuscule object and a workpiece utilizes an optical microscope and digital microscope using an optical lens, and the like. The digital microscope receives reflected light or transmitted light from an observation target fixed at an observation target fixing unit entering through an optical system with a light receiving device such as a CCD, which electrically reads the light for every pixel arranged two-dimensionally, and displays the electrically read image on a display unit such as a display (e.g., see Japanese Laid-Open Patent Publication No. 2002-135648).
The sensitivity region of the light receiving device such as a CCD is limited to a particular range, whereas the sensitivity range of a human eye is wider. Thus, the range that can be represented (dynamic range) of the image imaged with the CCD is limited compared to the human eye. For instance, the dynamic range is eight bits in a general JPEG image. In this case, if the tone of the image to be imaged exceeds such a range, saturation occurs, thereby causing underexposure, overexposure, halation, and the like. In order to solve such problems, used is a high dynamic range image (hereinafter referred to as “HDR image”) in which a plurality of low tone images are imaged with the dynamic range of the luminance region synthesized to obtain a high tone image. The HDR image is obtained by synthesizing a plurality of images at different levels of exposure of the same subject, and has a wide dynamic range from the darkest shadow (black) to an extremely bright highlight (white). For instance, a plurality of eight bit images is synthesized to generate a high tone HDR image of sixteen bits or thirty-two bits, and such an image is saved. Overexposure occurs at the metal portion in the image shown in FIG. 1, and underexposure occurs in the image shown in FIG. 2. When such images are synthesized, the HDR image as shown in FIG. 3 can be generated. The portions of overexposure and underexposure in the original image are clearly represented in the HDR image of FIG. 3.
When displaying the HDR image synthesized in the above manner on a monitor and the like, tone conversion (tone mapping) needs to be performed to a color range that can be displayed on the monitor, that is, a low dynamic range. Since, for example, only 16.77 million colors can be represented in twenty-four bit color, and only 256 colors in the case of eight bits in a general personal computer, the HDR image of thirty-two bits is handled as twenty-four bits or eight bits through tone mapping.
Various techniques have been proposed to widen the dynamic range using the HDR image. In order to resolve overexposure and underexposure by widening the dynamic range, photographing is performed in the range from darkness to an extent where overexposure does not occur in any region of the image, that is, with a short exposure time, to brightness to an extent where underexposure does not occur in any region, that is, with a long exposure time. Japanese Laid-Open Patent Publications Nos. 2002-135648 and 2002-223387, for example, disclose techniques of determining in which exposure time range a plurality of images be photographed, using temporary photographing or the like, as a technique of controlling the exposure time in photographing an original image for synthesizing an HDR image while changing the exposure time when widening the dynamic range.
Generally, such an HDR image is often used in applications to eliminate halation contained in the image data, correct backlight, and the like. In the application of magnification observation, there may also be a demand to observe fine patterns contained in a narrow dynamic range as opposed to the application of literally widening the dynamic range for halation measures and the like. In this case, tone images are finely imaged and synthesized in a limited dynamic range to enhance the S/N ratio and luminance resolution. Therefore, the setting of exposure times also differs from that in the dynamic range widening application. There have not been examples of magnification observation apparatus using change in dynamic range for such an application. Also, there has not been a magnification observation apparatus capable of switching between the dynamic range widening application and the luminance resolution enhancing application, and automatically determining in which application a user desires to photograph.